1. Field of the Invention
The present invention relates to a magnetic signal reproducing device, a magnetic signal reproducing method and a magnetic recording medium which implements high density reproduction by magnetically carrying out reproduction while raising the temperature of a recording medium.
2. Description of the Related Art
In recent years, a multimedia technology has been developed and a memory device having a larger capacity has been required to process information having a larger capacity. In particular, a high density recording technology mainly including a writable optical disk, a magnetic disk and a magnetic tape has been investigated vigorously.
There has been proposed a magnetic signal recording and reproducing method capable of carrying out high density recording and reproduction by locally providing a temperature raising area on a medium through light irradiation or the like using a magnetic recording medium having magnetic properties changed depending on a temperature and recording or reproducing only the temperature raising area selectively and magnetically (the method will be referred to as a thermal assist magnetic signal recording and reproducing method which will be hereinafter referred to as a thermal assist magnetic signal reproducing method in this application).
As an example of the thermal assist magnetic signal recording and reproducing method, there has been proposed a method of irradiating a light beam on an area of a recording medium where recording is to be carried out, raising a temperature to the vicinity of a Curie temperature and applying an external magnetic field by means of a recording head to record information during recording and of irradiating a light beam on an area of the recording medium where reproduction is to be carried out and raising a temperature to increase the magnetization of a reproducing portion, detecting a leaking magnetic flux by means of a reproducing head to reproduce the information during reproduction by using, as the recording medium, a ferrimagnetic substance having a temperature at which the magnetization is set to be zero in the vicinity of a room temperature (which will be referred to as a magnetic compensation temperature in this application) (Japanese Laid-Open Patent Publication No. Hei 4-176034).
In the conventional thermal assist magnetic signal reproducing method (Japanese Laid-Open Patent Publication No. Hei 4-176034), however, the magnetization is generated from all the areas in which the temperature of the recording medium is raised. Therefore, if the reproduction is to be carried out in a smaller area than the temperature raised area, the undesirable magnetization signal is mixed into a reproducing signal. As compared with a signal obtained in an area to be reproduced, the mixed signal should be small enough. For this reason, the reproduction area of an allowable range cannot be reduced.
More specifically, in the conventional thermal assist magnetic signal reproducing method, if a signal reproducing area width is smaller than a temperature raising area width in order to increase a storage density, the S/N ratio of the reproducing signal is decreased. A typical example will be described below.
It will be supposed that magnetized information (signal) recorded on a magnetic recording medium by causing negative and positive polarities of magnetizations to correspond to binary is reproduced by the thermal assist magnetic signal reproducing method using reproducing means (hereinafter referred to as a reproducing head) having a general rectangular magnetized information detecting area. FIG. 20 is a diagram typically showing a magnetized information detecting area 4 of the reproducing head and a temperature raising area 9 formed on a medium by means of a locally temperature raising device (for which a laser beam is used).
FIG. 20 shows magnetized information about three areas 5a, 5b and 5c which are magnetized into three different kinds of patterns and are recorded. A state in which the magnetic recording medium is magnetized with polarities which are alternately reverse to each other is represented by the direction of a hatching. A bit pattern in which a series of information are recorded with the polarity of the magnetization will be hereinafter referred to as a track 5. In this example, the magnetized information are present on three tracks, that is, one track (5b) in the center and one track (5a, 5c) at each side.
It will be supposed that the magnetized information of one track (5b) in the center of a medium having the magnetized information recorded thereon is reproduced by means of a reproducing head (for example, an MR head or the like) including the magnetized information detecting area 4 having a larger width than one track width in the direction of the track width.
It is assumed that the magnetic recording medium is kind of a n-type ferrimagnetic substance having a magnetic compensation temperature almost equal to a medium temperature in the vicinity of a portion provided under the reproducing head in a state in which the temperature is not raised. FIG. 21 shows the temperature characteristic of the magnetization of the magnetic recording medium The widths of the tracks 5a, 5b and 5c are almost equal to each other and distances between the adjacent tracks are almost equal to each other. The sum of the width of the track and the distance between the tracks will be referred to as a track pitch. The recording density of the magnetic recording medium is inversely proportional to the track pitch.
The temperature of the magnetic recording medium is almost constant in the thickness direction. A length in the track direction of the magnetized information detecting area 4 of the reproducing means is smaller than the spread of a temperature distribution. Therefore, the temperature is almost constant at the magnetically detectable area along the track direction. Only the temperature distribution in the track width direction in a reproduction-related area contributes to a reproducing signal. Hereinafter, only the distribution in the track width direction in the above-mentioned reproduction-related area will be used for the temperature distribution related to the detection of magnetized information.
In the case where only the central track 5b shown in FIG. 20 is reproduced by the thermal assist magnetic signal reproducing method, the temperature of only the central track 5b is raised through a light beam or the like. At this time, the light beam causes the magnetic recording medium to have a temperature distribution shown in FIG. 3. A magnetization is induced according to the temperature distribution. A portion of the magnetic recording medium in which the area 9 having the temperature raised overlaps the reproduction-related area scanned by the magnetized information detecting area 4 of the reproducing head for magnetization detection will be hereinafter referred to as a reproduction portion. If only the central track 5b is present in the reproduction portion, the magnetized information about only the central track 5b can be reproduced. When the temperature raising area 9 using the light beam and the magnetized information detecting area 4 of the reproducing head are moved along the track 5 with relative positions thereof almost fixed, all the magnetized information of the central track 5b can be detected.
FIGS. 22(a) and 22(b) are charts showing a width of the temperature raising area 9 in the track width direction and a difference in a magnetizing signal depending on the track width. FIG. 22(a) shows the case in which the width of the temperature raising area 9 is smaller than the track width, and FIG. 22(b) shows the case in which the width of the temperature raising area 9 is greater than the track width.
If the width of the temperature raising area 9 is smaller than the track width (FIG. 22(a)), the reproducing signal is almost occupied by a signal sent from the central track 5b and a mixed signal is rarely sent from the adjacent tracks 5a and 5c. On the other hand, in the case in which the width of the temperature raising area 9 is greater than the track width (FIG. 22(b)), a signal sent from the adjacent tracks 5a and 5c is greatly mixed in the reproducing signal. When the mixed signal becomes great to some extent, the magnetized information of the central track 5b cannot be reproduced correctly.
Accordingly, it is necessary to reduce the width of the temperature raising area 9 in the magnetic recording medium in order to decrease a track pitch while reducing the mixed signal for magnetized information reproduction with a higher density, which is very difficult. In the thermal assist magnetic signal reproducing method which has been conventionally proposed, it is impossible to decrease the track pitch while reducing the mixed signal sent from the adjacent tracks.
Also in a laser beam for converging and irradiating coherent light which is one of temperature raising means, a light intensity distribution thereof has a spread equal to or larger than a wavelength and a heat distribution has a greater width, which has been hard to suppress.
In the thermal assist magnetic signal reproducing method, moreover, the temperature of the magnetic recording medium facing the magnetized information detecting area 4 of the reproducing head during the reproduction should be almost equal to the magnetic compensation temperature during non-temperature raising. However, the temperature of an environment in which the device is put has a variation of several tens .degree.C. depending on seasons and time and electronic parts in the device generate heat. Moreover, heat is generated in a very small area, that is, the reproducing head generates heat. All the changes of heat influence the temperature of the magnetic recording medium. Therefore, in order to make the temperature of the reproduction-related area of the magnetic recording medium during the non-temperature raising almost equal to the magnetic compensation temperature, the temperature of a very small area, that is, the reproduction-related area of the magnetic recording medium of the reproducing head should be measured and controlled, which is very hard and cannot be solved at an actual cost.